The present invention relates to a two-wire bus system for connecting a plurality of users to at least one damping circuit for damping line resonances in the two-wire bus system with respect to a reference potential.
For transmitting information among different users in a motor vehicle, two-wire bus systems or so-called Controller-Area-Networks (CAN) are known. The hardware of a two-wire bus system of this type designed for high information-transmission speeds is composed of a two-wire bus line to which a plurality of users (transmitters/receivers) are connected. The two bus lines are typically both terminated at 120xcexa9. The function of the two-wire bus system lies in the fact that information signals among the users are transmitted as a voltage difference between the two bus lines, thus achieving greater signal reliability and less susceptibility to interference. In the state of rest, both bus lines (C_H, C_L), for example, are at a potential of 2.5 V. If a signal representing the logical xe2x80x9c1xe2x80x9d is sent over the two-wire bus line, then the potential of first bus line C_H is raised, for example, to 3.5 V, whereas the potential on bus line C_L is lowered to 1.5 V. This voltage difference is then detected at an input stage of a user.
If high-frequency radiation should arise, for example, as a result of a mobile radio or the like, resonance voltages, with respect to zero potential, can arise in the bus line or in portions thereof in the metal body of the motor vehicle. It is true that resonance voltages of this type regularly occur in both bus lines of the two-wire bus system without thereby significantly impairing the useful signal, which is the differential signal between the two bus lines. However, the resonance voltages can be so high that the input stage of a user is overloaded and therefore a reliable signal transmission is no longer possible. Furthermore, as a result of asymmetries in the bus system, resonance voltage interference can also negatively influence the useful signal as well.
One possibility for damping resonance voltages of this type in a two-wire bus system is a common-mode termination of the two ends of the bus system (see FIG. 1, resistor 12, 13, capacitor 14), in which the ends of the two bus lines are connected to each other via two serially connected resistors, for example, of 60xcexa9, and the connecting point of the serial resistors, which is arranged symmetrically with respect to the two bus lines, is connected to ground by a capacitor (optionally connected in series to a further resistor). In this manner, high-frequency resonance voltages are removed (diverted) to ground, the 120xcexa9-termination of the ends of the two bus lines being maintained. A common-mode termination of this type, however, is only possible at the two ends of the two bus lines. A common-mode termination within the bus line or in a branched line of same would lead to undesirable reflection effects in the bus line.
A further possibility to protect the receiver from overload from high-frequency resonance voltages lies in inserting a common-mode choke (choking code) between the two bus line inputs of a user. At both inputs of the user (to bus line C_H and to bus line C_L), a coil is attached, which is coupled to the other coil so that the resonance voltages cancel each other out. However, to provide a common-mode choke for each user is cumbersome and expensive. In addition, the upstream inductances, in connection with the capacitances in the user circuit, can lead to undesirable oscillations.
In a two-wire bus system according to the present invention the damping characteristic curve of the damping circuit is selected such that the damping only becomes operative above a preestablished resonance voltage threshold value. In this manner, a weakening of the useful signal is avoided. However, resonance voltages that lie above the threshold value and that could bring about an overload of the user input stages are damped.
The damping circuit is advantageously composed of a series circuit of a resistor having a voltage-limiting element and is connected, as a series circuit, between each of the two bus lines of the two-wire bus system and the reference potential. The voltage-limiting elements can be adjusted so that voltages in the range of the useful signals remain undamped, but voltage peaks above a voltage threshold value are diverted off via the resistor to ground.
The voltage-limiting elements are preferably configured so that the positive as well as negative half wave of a resonance oscillation is damped symmetrically with regard to a normal voltage potential (preferably 2.5 V) applied on each of the bus lines. In this manner, it is assured that an overload of user input stages is prevented both by positive as well as by negative half waves from resonance voltages. The voltage limit of the voltage-limiting elements, in this context, preferably lies 3 V above and below the normal voltage level of the two-wire bus system.
The voltage-limiting elements, for example, have Zener diodes for limiting the voltage.
It is advantageous if the reference potential is ground. The two-wire bus system according to the present invention can be designed, for example, for transmitting information in a motor vehicle.
According to one advantageous refinement of the present invention, each user connected to the two-wire bus system has assigned to it a damping circuit. In this manner, resonance voltages arising locally, for example, at dead-end segments of the bus line can be reliably damped. Since useful signals are not weakened, a multiplicity of damping circuits does not signify a worsening of the signal transmission in the two-wire bus system. It is advantageous if the damping circuit assigned to a user represents one component of an integrated circuit, which minimizes manufacturing costs.